The hemoglobin molecule is made of four polypeptide chains of two different kinds (alpha and beta). This structural arrangement enables the molecules to undergo conformational change during the oxygenation. The conformational change alters largely the affinity of the molecule for the ligand involved (O2) and results in the sigmoidal shape of the oxygen dissociation curve which means easy release of oxygen to the tissues (cooperativity). The conformational change does not affect only the oxygenation but also other properties of the molecule such as oxidation. The (T) conformation being oxidized with a rate which is two order of magnitude larger than that of the (R) conformation. I have shown during the past five years that the alpha chains autoxidize 8-10 times more rapidly than the beta chains, but they have a higher affinity for oxygen. This in turn will shift the conformational equilibrium towards (R) form (protection of alpha chains against oxidation). The objective of the present study is to continue my investigation in the same line and study: 1. Purification of methemoglobin reductase by affinity chromatography. 2. The reduction of methemoglobin by NADH-dependent methemoglobin reductase (the main reducing enzyme in the red cell) for the following purposes: a. To investigate the effect of conformational change on the reduction reaction. b. To investigate the mode and nature of this enzymatic reaction. 3. The effect of ligand binding of subunits on the conformation of hemoglobin molecule in tetramic form.